In recent years, a variety of projectors have been widely used in various video applications. For example, projectors can be used for making presentations, holding meetings or giving lectures in classrooms, boardrooms, conference rooms or home theaters. By the projector, an image signal from an image signal source can be enlarged and shown on a display screen. For reducing power consumption and overall volume, the illumination system of the current projector employs a solid-state light-emitting element (e.g. light emitting diode or laser diode) to replace the conventional high intensity discharge (HID) lamp.
Generally, the illumination system of the projector may emit three primary color lights, i.e. red light (R), green light (G) and blue light (B). Among three primary color solid-state light-emitting elements including a red solid-state light-emitting element, a green solid-state light-emitting element and a blue solid-state light-emitting element, the blue solid-state light-emitting element has the highest luminous efficiency. Since the red solid-state light-emitting element and the green solid-state light-emitting element have poor luminous efficiency, the red light or the green light may be produced by using a blue solid-state light-emitting element and a wavelength conversion device (e.g. a phosphor wheel). That is, the uses of the blue solid-state light-emitting element and the phosphor wheel may directly emit the red light or the green light in replace of the red solid-state light-emitting element or the green solid-state light-emitting element. Consequently, the luminous efficiency of the whole illumination system is enhanced and the manufacturing cost of the illumination system is reduced.
Generally, the conventional illumination systems of the projectors are classified into two types. A conventional illumination system utilizes a single blue solid-state light-emitting element and a single phosphor wheel with plural sections. FIG. 1A schematically illustrates the architecture of a conventional projector. FIG. 1B schematically illustrates a phosphor wheel used in the illumination system of the projector as shown in FIG. 1A. As shown in FIGS. 1A and 1B, the illumination system of the projector 1 employs a solid-state light-emitting element 11 to emit blue light to a phosphor wheel 12 with a first section 121, a second section 122 and a third section 123. The first section 121 is coated with a green phosphor agent. By the green phosphor agent, the incident blue light is converted to green light. The second section 122 is coated with a red phosphor agent. By the red phosphor agent, the incident blue light is converted to red light. The third section 123 is a transparent section. The blue light is transmitted through the third section 123. In other words, the blue light from the solid-state light-emitting element 11 is directly transmitted through the phosphor wheel 12 or converted into the green light or the red light by the phosphor wheel 12. Consequently, three primary color lights can be produced. Moreover, the three primary color lights are directed to an imaging device 14 through a relay module 13. For example, the imaging device 14 is a digital micromirror device (DMD), a liquid crystal display (LCD) device or a liquid crystal on silicon (LCOS) device. After being scaled up/down and focused by a lens group 15, an image is projected on a display screen 16.
Another conventional illumination system utilizes three blue solid-state light-emitting elements and two phosphor wheels, wherein each of the two phosphor wheels is coated with a single color phosphor agent. FIG. 2A schematically illustrates the architecture of another conventional projector. FIG. 2B schematically illustrates a first phosphor wheel used in the illumination system of the projector as shown in FIG. 2A. FIG. 2C schematically illustrates a second phosphor wheel used in the illumination system of the projector as shown in FIG. 2A. Please refer to FIGS. 2A, 2B and 2C. In the conventional illumination system of the projector 2, a section 221 of a first phosphor wheel 22 is coated with a red phosphor agent, and a section 241 of a second phosphor wheel 24 is coated with a green phosphor agent. By the red phosphor agent, the incident blue light is converted to red light. By the green phosphor agent, the incident blue light is converted to green light.
The projector 2 further comprises a first dichroic mirror 210 and a second dichroic mirror 211, a first solid-state light-emitting element 21, a second solid-state light-emitting element 23, and a third solid-state light-emitting element 25. The red light is permitted to be transmitted through the first dichroic mirror 210, but the green light is reflected by the first dichroic mirror 210. The red light and the green light are permitted to be transmitted through the second dichroic mirror 211, but the blue light is reflected by the second dichroic mirror 211. The blue light from the first solid-state light-emitting element 21 is converted to red light by the first phosphor wheel 22. The red light is transmitted through the first dichroic mirror 210 and the second dichroic mirror 211 and directed to a relay module 26. The blue light from the second solid-state light-emitting element 23 is converted to green light by the second phosphor wheel 24. The green light is sequentially reflected by the first dichroic mirror 210, transmitted through the second dichroic mirror 211 and directed to the relay module 26. The blue light from the third solid-state light-emitting element 25 is reflected by the second dichroic mirror 211 and directed to the relay module 26. Moreover, the three primary color lights are sequentially or simultaneously directed to an imaging device 27 through the relay module 26. After being scaled up/down and focused by a lens group 28, an image is projected on a display screen 29.
From the above discussions, the uses of the blue solid-state light-emitting element and the phosphor wheel may directly emit the red light or the green light in replace of the red solid-state light-emitting element or the green solid-state light-emitting element. However, since the green light converted by the green phosphor agent contains a portion of red light, the green light looks somewhat yellowish. That is, the color purity is insufficient, and thus the imaging quality is impaired. Moreover, the exciting efficiency of red phosphor is lower and easier saturated than the green phosphor, the total amount of red light converted from the red phosphor agent is insufficient. As the driving current of the blue solid-state light-emitting element increases, the red light converted by the red phosphor agent quickly saturates or even decay. Under this circumstance, the luminance and brightness of the red light is too low, and the bright/dark status of the illumination system fails to be effectively controlled. Consequently, the overall amount of the output light is limited.
In addition, in a reflective phosphor wheel, the reflectivity and the reflection spectrum of which are the key to decide the capability of the phosphor wheel. The general reflective coatings are usually made of silver or aluminum for covering all the range of the visible light. Please refer to FIG. 3. FIG. 3 schematically illustrates the reflectivity of silver and aluminum corresponding to visible light with wavelength between 400 and 700 nanometers and the phosphor spectra of green light, yellow light and red light. Since the chemical stability of silver is relative lower, the gathering and sulfation phenomena of silver atom are occurred when the power of Laser or the operation temperature is high, and further the reflectivity is significantly decreased. Under this circumstance, a phosphor wheel applied under high energy usually utilizes aluminum as the reflective coating. Although aluminum is relative more stable, the reflectivity of itself is lower, especially lowest at the red light waveband with wavelength between 600 and 700 nanometers. As a result, a phosphor wheel utilizing aluminum as the reflective coating has the issue of insufficient output of red light causing the decreasing of output efficiency. In brief, no matter using silver or aluminum as the reflective coating, the performance of reflectivity is not actually well.
Therefore, there is a need of providing an improved phosphor device that provides max outputs of each waveband in order to eliminate the above drawbacks.